This invention relates to the chemical displacement plating. More particularly, this invention relates to plating of tin on copper, copper alloys, and other metals by chemical displacement using a spray or cascade application process. Still more particularly, this invention relates to the use of such chemical displacement plating in the manufacture of printed circuit boards.
Coatings of tin typically have been applied to surfaces of copper and copper based alloys by a particular mode of displacement plating, i.e., immersion plating techniques such as disclosed in U.S. Pat. No. 2,891,871, U.S. Pat. No. 3,303,029 and U.S. Pat. No. 4,715,894. ("Displacement" plating has also been referred to as "replacement" plating and for the purpose of this invention the terms are intended to be synonymous.) In the disclosed immersion tin plating techniques, a bath is prepared containing an aqueous solution of a tin(II) salt, an acid, and thiourea or a thiourea derivative as essential ingredients. In the immersion tin plating process, an article bearing a copper surface, e.g., a copper clad printed circuit board, is immersed in the plating bath for a period of time during which the surface copper metal is oxidized to copper(I) ion and complexed with the thiourea and is replaced at the surface by the concurrently reduced tin metal from the tin(II) ion. After displacement plating has been completed to a desired thickness, the article is removed from the bath and is rinsed to remove residual plating solution. During the plating process the concentration of copper(I) thiourea complex in the immersion bath increases. Likewise, some aerial oxidation of tin(II) ion leads to increased tin(IV) ion concentration during the life of the plating bath. However, the concentrations of copper(I) complex and tin(IV) ion rapidly equilibrate due to the substantial drag-out of the plating solution with the plated article and the subsequent bath replenishment. The presence of tin(IV) ion in tin displacement plating is undesirable since it reduces the efficiency of the plating bath. Immersion plating baths typically have a very small surface-to-volume ratio which minimizes aerial oxidation and typically the equilibrium concentration of tin(IV) ion is within acceptable limits. Nevertheless, when plated surface thickness is critical, as in some printed circuit board applications, undesirable aerial oxidation during removal of the article from the immersion bath can result in streaks of non-uniform thickness in the plated surface.
Chemical displacement plating has been used in the manufacture of printed circuit boards (PCB's) and particularly multilayer printed circuit boards. Printed circuit boards (PCB's) comprise a non-conducting or dielectric such as a fiberglass/epoxy sheet which is clad with a metal conductive layer such as copper on either one or both surfaces. The metal layer on the PCB before processing typically is a continuous layer of copper which may be interrupted by a pattern of plated through holes or vias linking both surfaces of the board. During processing selected portions of the copper layer are removed to form a raised copper circuit image pattern of the PCB. Multilayer PCB's are typically constructed by interleaving imaged conductive layers such as one containing copper with dielectric adhesive layers such as a partially cured B-stage resin, i.e., a prepreg, into a multilayer sandwich which is then bonded together by applying heat and pressure. Production of these types of printed circuit boards are described in "Printed Circuits Handbook", Third Edition, edited by C. F. Coombs, Jr., McGraw-Hill, 1988, which is incorporated herein by reference. Since a conductive layer with a smooth copper surface does not bond well to the prepreg, several copper surface treatments have been developed to increase the bond strength between the layers of the multilayer PCB sandwich.
One such copper surface treatment is the use of immersion tin and tin alloys as a bonding medium for multilayer circuits as disclosed by Holtzman et al., U.S. Pat. No. 4,715,894. In the disclosed process an immersion tin composition is disclosed containing both thiourea compounds and urea compounds to displacement plate the copper surface of each PCB with tin by the immersion process prior to laminating them to form a multilayer board. Although bond strength of multilayer PCB's prepared by this immersion process was improved, the production efficiency of multilayer PCB's is limited by the batch process wherein substantial quantities of plating bath is lost through drag-out of the solution with each PCB processed. Moreover, the PCB's made by this immersion process are susceptible to defects due to streaking described supra.
Innerlayer bonding of multilayer PCB's has been further improved by the process disclosed in assignee's U.S. patent application, Ser. No. 07/446335, filed Dec. 5, 1989, now U.S. Pat. No. 5,073,456 and in a publication in "Printed Circuit Fabrication", Vol. 13, No. 5, pages 46-60, May 1990 by K. H. Dietz, J. V. Palladino and A. C. Vitale, entitled MULTILAYER BONDING: CURRENT TECHNOLOGY AND A NEW ALTERNATIVE. The in-line process disclosed includes a spray displacement tin plating step followed by a post-treatment step with a silane bonding mixture of a ureido silane and a disilyl crosslinking agent. In particular, PCB's are fed by conveyor through a series of treatment and rinse stations in which the PCB's are sequentially cleaned, microetched, spray tin displacement plated, post-treated with the silane bonding mixture and dried. The PCB's prepared by this spray tin displacement plating system are substantially free of streak defects observed in the immersion batch process and the multilayer PCB's prepared therefrom demonstrate improved resistance to delamination during typical high temperature soldering operations. During the plating process the plating solution is sprayed onto the PCB and the excess solution is recovered and returned to the plating bath sump with minimal drag-out to succeeding rinse stations. Although improved multilayer PCB's have been obtained by the disclosed process, it has now been observed that the activity of the plating bath solution declines during use due to the accumulation of tin(IV) ion formed by aerial oxidation of tin(II) during the spray application step. Concurrently, the concentration of copper(I) thiourea complex increases in the recirculated plating solution until its solubility limit is surpassed and crystalline complex is precipitated which clogs the spray nozzles and interferes with the mechanical components of the plating system. In order to take full advantage of the benefits of the spray tin displacement plating process, there is a need to stabilize the activity of the tin plating bath and eliminate the insoluble copper(I) thiourea complex precipitate and thereby extend the life of the plating bath with minimal replenishment.